This invention relates to anodized aluminum and more particularly to a method of providing a low temperature, smudge free seal on anodized aluminum.
Because of the need to conserve energy resources, considerable effort has been expended in finding new methods for sealing anodized aluminum in ways which would reduce the energy requirement necessary for such sealing.
Anodized aluminum is usually sealed to render it impervious to elements which could adversely affect the aluminum substrate, because, in many instances, especially in architectural applications, it will be exposed to the atmosphere for many years. It is therefore imperative that the seal be of very high quality to ensure satisfactory weatherability.
High quality seal as used herein, is defined as a sealed, smudge-free anodized aluminum, which, after being treated with a standard chromic/phosphoric acid solution known to those skilled in the art as the "acid dissolution test," has a weight loss of not more than 2 mg/in.sup.2.
In the prior art, in order to effect a satisfactory seal on anodized aluminum, it was necessary to maintain the sealing solution at or near the boiling point. This high temperature was necessary to provide a high quality seal such that subsequent subjection of the sealed anodized aluminum to a mineral acid treatment to remove sealing smudge provided a smudge-free, sealed anodized surface. For example, Alexander U.S. Pat. No. 3,791,940 teaches sealing undyed anodized aluminum in aqueous hydrolyzable metallic salt at about 212.degree. F. or boiling. Alternatively, Wallace U.S. Pat. No. 3,822,156 discloses sealing anodized aluminum in a solution of triethanolamine in hot water, with a preferred temperature being in the range of 175.degree. F to about 212.degree. F with optimum being near the boiling range of the solution. In both patents, it is taught that the sealed anodized aluminum is thereafter desmudged with mineral acid. Meyer et al. U.S. Pat. No. 3,897,287 discloses the addition of a controlled amount of a soluble sulfate (SO.sub.4 .sup.=) to a hydrolyzable salt such as disclosed and claimed in the aforesaid Alexander patent to provide a smoother sealing smudge which is lesser in quantity and is easier to remove in the subsequent mineral acid treatment. Again, it is suggested that the sealing bath temperature should be hot with a temperature range of 195.degree. to 212.degree. F. being preferred. This process for sealing and subsequently removing sealing smudge with mineral acid has found widespread acceptance in the anodizing industry particularly for architectural applications.
As previously stated, the anodized surface must be properly sealed. To obtain such a seal, it can be seen from the prior art (such as that referred to above) that it was thought that a hot sealing bath, preferably at or near the boiling point of the sealing bath, was essential, especially when the sealed surface was subsequently subjected to a mineral acid treatment to remove smudge formed during sealing.
Quite surprisingly, we have discovered a method for sealing anodized aluminum at a lower temperature wherein sealing smudge formed thereby can still be advantageously removed with mineral acid without interfering with the quality of the seal.